This invention relates to packaging machines of the form-fill-seal type adapted to form a flexible thermoplastic film into a cylindrical tubular shape and thermally seal it after objects to be packaged are deposited inside. This invention relates more particularly to methods of and means for detecting objects where portions of the film are sealed together to close a bag formed by such a packaging machine.
Packaging machines, for example, of a vertical pillow type as disclosed in U.S. Pat. Nos. 5,279,098 issued Jan. 18, 1994 and 5,347,795 issued Sep. 20, 1994 (which are herein incorporated by reference) are adapted to form an elongated film into a cylindrical shape and seal it transversely as objects to be packaged are dropped inside. In order to prevent any of these objects from remaining where portions of the cylindrically formed film are sealed together, such packaging machines are usually adapted to provide vibrations to the film so as to shake and thereby remove crumb and small pieces of the objects from the inner surfaces of the cylindrically formed film where the sealing takes place. Measures have also been taken to force the film into taking a form with a rounded bottom in order to prevent the film from becoming wrinkled such that the film strength and air-tightness at sealed positions will not adversely affected.
In spite of all such measures taken, however, objects being packaged are sometimes caught where the transverse sealing takes place, and the film becomes wrinkled not infrequently. For this reason, it has been known to use a photoelectric device to detect the gap between sealing heaters or to detect variations in the pressure in the film in order to determine whether an abnormal condition has developed in the sealed position.
In general, however, it is not always at the same position that objects being packaged are caught or the film becomes wrinkled. Moreover, there are local variations in the thickness of the film. Thus, prior art detecting means of the kind described above cannot always accurately detect the existence of crumbs or small pieces at the seal positions. If the film thickness changes from one lot to the next, in particular, prior art detecting means are required to be reset every time a roll of film from a new lot is installed.
It is therefore an object of the present invention to provide improved methods of and means for accurately detecting the existence of objects inserted where a tubularly formed film is transversely sealed together as a packaging machine of a form-fill-seal type is operated to make packages.
It is another object of the invention to provide packaging machines adapted to use such an improved detection method.